U.S. President Joe Biden on Saturday recognised the 1915 killings of Armenians by Ottoman forces as genocide, a watershed moment for descendants of the hundreds of thousands of dead as he defied decades of pressure by Turkey.

Mr. Biden became the first U.S. President to use the word genocide in a customary statement on the anniversary, a day after informing Turkish President Recep Tayyip Erdogan that he would go ahead with this step. “We remember the lives of all those who died in the Ottoman-era Armenian genocide and recommit ourselves to preventing such an atrocity from ever again occurring,” Mr. Biden said.

“And we remember so that we remain ever-vigilant against the corrosive influence of hate in all its forms,” he said.

The statement is a massive victory for Armenia and its extensive diaspora. Starting with Uruguay in 1965, nations including France, Germany, Canada and Russia have recognised the genocide but a U.S. statement has been a paramount goal that proved elusive under other presidents until Mr. Biden.

Mr. Biden said his statement was “not to cast blame but to ensure that what happened is never repeated.”

As many as 1.5 million Armenians are estimated to have been killed from 1915 to 1917 during the waning days of the Ottoman Empire.